Study On The Combined Measuring Method Of HF And UHF Used For Monitoring PD Of Transformer

High frequency (HF) and ultra high frequency (UHF) measurements are adoptedin on-line partial discharge (PD) monitoring for power transformer. The HF method isin accord with the standard, and we can get the apparent discharge quantities with it.But it also has some disadvantage such as poor interference influences ability, so thatit is hard to achieve high sensitivity. While the UHF method has better interferenceinfluences ability, but it can only measure the relativity variety of the discharge, andthis is limited its use. Obviously, if we unite them to use their strong points to offsettheir weakness, we will get an efficient method to improve the sensitivity of on-linePD monitoring.The sensitivity of on-line PD monitoring for transformer has been improvedhardly because of the corona discharge in air. In order to confirm the measurefrequency band of the UHF measure devises for restricting corona interference, weneed to know the frquency characteristic of the discharge signals in air and in oil.We designed an Archimedean spiral antenna to receive the UHF signals, andbuild the UHF detection system with the TDS-684C digital oscilloscope to meet theneed of the laboratory study. In the laboratory, we studied the frequency characteristicof the discharge signals in air and oil. The results show that the frequency ranges ofPD in oil from 250MHz to 1GHz, and frequency ranges of PD in air is under200MHz. And then we can take the 300MHz as their "point of frequency boundary".The spread characteristic of the UHF signals in metal tank and free space wasstudied. It shows that the attenuation of UHF signal in metal tank is slow than that inair. Due to the reflection and refraction, the amplitudes of UHF signal measured atdifferent positions are quite different. So the UHF method can only get the relativechange of the partial discharge signals.We also designed the practical UHF detect unit (PUDU) including the pull-rod,the UHF sensor, the UHF amplifier, the UHF filter and the UHF demodulation cell.The demodulation circuit can acquire the HF components of the UHF signals, whichwill reduce the sampling rate of the A/D card.Uniting the PUDU with the HF partial discharge monitoring system fortransformer, we can get a practical combine monitoring system. The experimentswere carried out with the new monitoring system to study the model discharge inlaboratory. The results show that the PD signals detected with the HF method arecorrespond with that detected with the UHF method in time domain. And we canremove the interference of the HF signals after simple signal processing to improvethe ratio of the signal to noise. At the same time, we can obtain the apparent dischargequantities by the HF detecting.